The present invention relates to the pinion drive shaft of a refrigeration compressor. In particular, the present invention is directed to the coupling between the pinion drive shaft and one or more impellers in either a direct drive or gear drive centrifugal compressor.
Prior to this invention, the pinion drive shafts of refrigeration compressors have had generally circular ends, and have been splined or keyed in one or more places to facilitate their connection to one or more impellers of the compressor.
Different sized pinion drive shaft couplings are beneficial to optimal performance of a refrigeration compressor. But the spline shaft portions used in the prior art are not easily machined to different sizes on the same shaft.
In addition, splined or keyed shafts are susceptible to high stress concentrations due to the stress risers inherent in the multi-faceted and intricately machined splines and keys. For example, consider a small-diameter portion of a shaft which steps to a larger diameter and has a spline on the small-diameter portion. To accommodate hob runout at the end of each flute of the spline, either the small-diameter portion must extend beyond the spline, causing the shaft to be weaker, or the hob runout must extend into the larger-diameter shaft portion. This latter accommodation is more difficult and also weakens the larger-diameter shaft portion.
It is beneficial for optimal refrigerant gas flow and overall performance of the compressor to have the pinion shaft-impeller coupling nearest the refrigerant gas inlet have as small a cross-sectional area as possible. By the same token, in order to raise the natural frequency of the system, thus making the compressor more stable and balanced, it is important that the pinion shaft-impeller couplings for the rest of the impellers in the compressor have increasingly larger cross-sectional areas. Similarly, as the compressor increases in size, the pinion shaft size is increased, thereby also increasing the size of the cross-sectional area of the shaft-impeller coupling.
A German standard, DIN 32711, for machining "driving components," particularly drive shafts, discloses a non-circular, radially lobed drive shaft portion that fits into a bore of complementary shape in a driven component to couple the shaft and driven component. However, refrigeration compressors are not believed to have employed such a lobed drive shaft, particularly in two-stage or multi-stage compressors having two or more such couplings on a stepped shaft.